The present invention relates a balanced dipole antenna, and more particularly, is directed to a symmetric balun used with a coaxial cable and dipole antenna.
FIG. 1 shows dipole antenna 10 as having coaxial cable 5 having outer coaxial conductor 15 and inner coaxial conductor 16 used with a dipole antenna having dipole left blade 11 and dipole right blade 12. Coaxial outer conductor 15 is connected to dipole right blade 12. Coaxial inner conductor 16 is connected to dipole left blade 11 via wire 17.
As used herein and in the claims, “coupling” includes a radiative connection and a direct electrical connection.
Since an isotropic antenna is physically impossible, antenna gain is measured against a standard dipole antenna, and the results are indicated as decibels vs. dipole (dBd).
Common mode current flows on the outside of the coaxial line, reducing the efficiency of a pure dipole radiation pattern. Additionally, common mode current is caused by radiative coupling between the dipole antenna and an external coaxial cable. The majority of the distortion of the dipole antenna pattern is due to common mode current flow caused by the conducting imbalance of the structure, and a smaller amount of the distortion is due to radiative coupling.
To reduce the common mode current flow, a balun is used. A balun acts as a transformer, connecting a balanced two-conductor line to an unbalanced coaxial line.
FIG. 2 shows dipole antenna 30 as having coaxial cable 5 connected to a dipole antenna using Roberts balun 40. The dipole antenna forms a balanced load (or source). Coaxial cable 5 connects to an unbalanced source (or load) and is connected to Roberts balun 40 at connection 41 which may be a threaded screw-type connection. Roberts balun 40 has a main coaxial segment having outer coaxial conductor 45 and inner coaxial conductor 46. Coaxial outer conductor 45 is connected to dipole right blade 32. Roberts balun 40 also has a short coaxial cable segment having outer conductor 35 and inner conductor 36. Roberts balun 40 is a quarter wavelength current choke. Coaxial outer conductor 35 is connected to dipole left blade 31. Coaxial inner conductors 35 and 45 are connected at their top ends via wire 37 and coupled to dipole left blade 31.
Sliding bar 38 connects the bottom end of coaxial outer conductor 35 to coaxial outer conductor 45. Sliding bar 38 creates a short circuit, providing an infinite impedance across the terminals of dipole left arm 31 and dipole right arm 32.
FIG. 3 shows dipole antenna 50 as having coaxial cable 5 coupled to a dipole antenna using IEEE-type balun 50, sometimes referred to as a Type III balun. The dipole antenna forms a balanced load (or source). Coaxial cable 5 connects to an unbalanced source (or load) and is connected to IEEE-type balun 50 at connection 51 which may be a threaded screw-type connection. IEEE-type balun 50 has a main coaxial segment having outer coaxial conductor 65 and inner coaxial conductor 66. Coaxial outer conductor 65 is electrically connected to dipole right blade 52. Coaxial inner conductor 66 is electrically connected to dipole left blade 51 via wire 57. IEEE-type balun 50 also has conductor 55 electrically connected to dipole left blade 51. IEEE-balun 50 is a quarter wavelength current choke. Conductor 55 is located generally parallel to the coaxial cable. Sliding bar 58 connects the bottom end of conductor 55 to coaxial outer conductor 65.
Sliding bar 58 creates a short circuit, providing an infinite impedance across the terminals of dipole left arm 51 and dipole right arm 52.
The quarter wavelength current choke in each of FIGS. 2 and 3 serves to reduce common mode current. However, conventional baluns used with dipole antennas do not prevent radiative coupling between a coaxial cable and the dipole antenna and do not completely eliminate common mode current. Accordingly, there is room for an improved coupling between a coaxial cable and a dipole antenna.